The present invention relates in general to telecommunication techniques. More particularly, the invention provides a system and method for generating optical return-to-zero signals with differential bi-phase shift and frequency chirp. Merely by way of example, the invention is described as it applies to optical networks, but it should be recognized that the invention has a broader range of applicability.
Telecommunication techniques have progressed through the years. As merely an example, optical networks have been used for conventional telecommunications in voice and other applications. The optical networks can transmit multiple signals of different capacities. For example, the optical networks terminate signals, multiplex signals from a lower speed to a higher speed, switch signals, and transport signals in the networks according to certain definitions.
In optical communications, an optical signal may transmit a long distance, such as hundreds or even thousands of kilometers, in optical fiber links. The quality of received signals often can be improved by using return-to-zero (RZ) modulations instead of non-return-to-zero (NRZ) modulations. For example, a signal under return-to-zero modulation includes logic low and high states, such as ones represented by “0” and “1” respectively. The signal state often is determined by the voltage during one part of a bit period, and the signal returns to a resting state during another part of the bit period. As an example, the resting state is represented by zero volt. In another example, a signal under non-return-to-zero modulation includes logic low and high states, such as ones represented by “0” and “1” respectively. The signal state often is determined by the voltage during a bit period without the signal returning to a resting state during at least a part of the bit period.
The return-to-zero modulations usually can provide better resistance to signal noises than the non-return-to-zero modulations. Additionally, the isolated RZ pulses often experience nearly identical nonlinear distortions during transmission, which can be at least partially mitigated through proper dispersion compensation schemes. Hence RZ signals usually are more resistant to nonlinear distortions than NRZ signals.
Among complex RZ signals, the optical carrier-suppressed return-to-zero (CSRZ) signals can provide strong transmission capabilities. For example, the CSRZ signals have alternating bi-phase shifts between adjacent bits, and are less affected by inter-symbol interference than the simple RZ signals, which often are intensity modulated without phase modulation. Thus the CSRZ signals are more tolerant for both dispersions and nonlinear distortions.
FIG. 1 is a simplified conventional system for generating CSRZ signals. The system 100 includes an NRZ source 110, an NRZ data driver 120, a CW diode laser 130, a data modulator 140, a clock driver 150, a phase shifter 155, and a clock modulator 160. The data modulator 140 and the clock modulator 160 each are an EO modulator. The EO modulator 160 is biased at null and driven by a half-rate data clock signal generated by the clock driver 150. In response, the EO modulator 160 can generate optical clock pulses. As shown in FIG. 1, the conventional system 100 for generating CSRZ signals often is complex and expensive.
Hence it is highly desirable to improve techniques for generating return-to-zero signals.